1. Field of the Invention
The present invention relates to a charged particle trap for X-ray analysis so as to analyze a specimen and its constituent elements by sending a beam of charged particles to strike the specimen and detecting the characteristic X-rays from the specimen with least concern about any undesired charged particles.
2. Description of Related Art
A method for analyzing a specimen""s elements by sending a beam of charged particles to strike the specimen and detecting the characteristic X-rays emitted from the specimen is known. An example of such method is the X-ray spectrometry of a spatial energy dispersive type in which the composition of a specimen is measured by detecting X-rays emitted from the specimen. This method is advantageous in that the characteristic X-rays from the specimen have energy equal to the electronic transitional energy of the elements constituting the specimen. The measurement of the volume of X-ray emissions per unit time for each X-ray provides information about how the specimen under examination is composed of elements. A semiconductor detector having semiconductor crystal such as silicon, germanium, etc. is generally used for detecting X-rays.
In recent years, as reported in Physics Today, July, 1998, pp. 10-21, an X-ray detector called a micro-calorimeter, which operates at ultra-low temperature (below 100 milli-Kelvin), has been developed. As compared with the above semiconductor detector, the micro-calorimeter detects the X-rays at high-energy resolution.
Along with the above-mentioned X-ray spectrometry of a spatial energy dispersive type, a method called X-ray spectrometry of a wavelength dispersive type using the combination of an X-ray spectrometer and a proportional counter is also known.
The schematic structural drawing of a typical radiation detector using the above-mentioned semiconductor detector is shown in FIG. 16. For noise reduction purposes, arrangement is made such that an X-ray crystal 101 and a field effect transistor 2 at the input stage of a pre-amplifier circuit block 20 is cooled to low temperature by a cryostat 7 using liquid nitrogen or a Peltier element in conjunction with cooling rods 12. When a beam of electrons 5 strikes a specimen 9, X-rays 1 are emitted from the specimen. The X-rays 1 pass through an X-ray window 8 and arrive at the X-ray crystal 101 where they are transformed into positive hole pairs of electrons proportional to the X-ray energy.
A method for processing signals produced by the X-ray crystal 101 will be described below. Electrons that reach at the electrodes of the X-ray crystal further travel through the pre-amplifier circuit block 20 of a charge integral type where they are transformed into pulses of voltage 220 with a height proportional to the number of the electrons. Furthermore, the pulses of voltage 220 are filtered through a shaping amplifier 51 so as to be shaped into pulses of voltage 310. The pulses of voltage 310 are inputted to a pulse height analyzer 53 where they are subjected to pulse height analysis and mapped in an X-ray spectrum 400. The X-ray spectrum 400 represents an energy distribution curve of the incident X-rays 1 detected by the X-ray crystal, i.e., how many X-rays of a certain energy level have been detected. The value of energy at a peak of the spectrum determines what is an element (a component) of the specimen and the count of the X-rays forming the spectrum peak determines the quantity of the element content.
Although, in the above description, attention is directed to the X-rays 1 emitted from the specimen 9 when the beam of electrons 5 strikes the specimen 9, reflected/back-scattered electrons 4 with a diversity of energy less than the energy of the incident electron beam also radiate from the specimen 9 by elastic or inelastic scattering. Such reflected electrons 4 are called backscattered electrons. When the backscattered electrons 4 are detected by the X-ray crystal as the incident rays, they are also transformed into electric signals as for the X-rays and cause background noise. Moreover, the backscattered electrons cause a defect in the X-ray crystal and significantly deteriorate the X-ray detecting performance of the X-ray crystal.
For the reason, a backscattered electron trap 3 is installed between the X-ray crystal 101 and the specimen 9, as shown in FIG. 16, so as to trap the backscattered electrons 4 from entering incidentally. Furthermore, as the backscattered electrons 4 strike against the surface of an object other than the specimen 9, X-rays are produced and cause more background noise.
Thus, a chamber 6 containing the X-ray crystal 101 is partially made of metal material to be thick enough to attenuate the X-rays so that the X-rays reflected from any objects other than the specimen will not enter into the X-ray crystal incidentally.
Another detector implementation mechanism is to keep the X-ray detector away from the specimen when the X-ray detection is not performed to further prevent the detector from being deteriorated by the backscattered electrons. Hereinafter, the backscattered electron trap will be referred to as an electron trap.
As described in Microscopy and Microanalysis, Vol. 4 (1999) pp. 605-615, for conventional X-ray detectors, a single X-ray crystal is generally used as the detector for detecting X-rays. The electron trap essentially consists of a pair of permanent magnets.
Another conventional electron trap is shown in FIGS. 17, 18, and 19. This trap has a structure similar to the trap disclosed in JP-A-103379/1981. FIG. 17 shows a section of the trap including the optical axis and the axis of X-ray detection. FIG. 18 shows another section of the trap taken along the line XVIIIxe2x80x94XVIII in FIG. 17. FIG. 19 shows yet another section of the trap taken along the line XIXxe2x80x94XIX in FIG. 18.
The electron trap 300 includes two permanent magnets 21 and 22 placed above and under a X-ray path hole 11, a cylindrical support 15 made of soft iron with a magnetic path 13 and a groove 14, and a cover 16. A magnetic field 17 is generated in the X-ray path hole 11 so as to turn the incident backscattered electrons 4 to the hole 11 toward a direction perpendicular to the direction in which the electrons 4 travel and the direction of the magnetic field 17 by the Lorentz force. This causes the backscattered electrons to strike against the walls of the groove 14 so that they will not enter the X-ray crystal incidentally.
The groove 14 is formed such that X-rays 18 emitted when the backscattered electrons 4 strike against the walls of the groove 14 as shown in FIG. 19 will not enter the X-ray crystal 101 (see FIG. 16) incidentally. The magnetic flux density of the magnetic field is a few times larger than 0.1 tesla. If the magnetic flux density of the magnetic field is 0.2 tesla, electrons with energy of 20 keV incoming perpendicular to the magnet field is redirected to curve with the radius of curvature of about 2 millimeters.
The conventional X-ray detection by using a single X-ray detecting element has been illustrated above, FIGS. 20 and 21 illustrative an X-ray detector implementing a plurality of X-ray detecting units as disclosed in JP-A-222172/1996.
FIG. 20 shows the X-ray detector setup on an electron microscope, while four X-ray crystals 101 are set symmetrically on the right and left sides. These X-ray crystals 101 detect characteristic X-rays emitted from the specimen 9 when the beam of electrons 5 strikes the specimen. Reference numeral 107 denotes a collimator for reducing X-rays that travel in random directions.
FIG. 21 is an enlarged perspective view of the collimator shown in FIG. 20 for explaining two electron traps provided in the collimator for the innermost two X-ray crystals 101. The collimator 107 is made of tantalum and has an electron beam path hole 700 through which the electron beam 5 is allowed to travel along its axis. The hole is in the shape of a frustum of circular cone. There are a pair of X-ray path holes 74 and a pair of permanent magnets 270. The pair of permanent magnets 270 and a beryllium plate 72 are fit to the inner surfaces of each hole.
When the beam of electrons 5 strikes a specimen 9, characteristic X-rays 79 emitted from the specimen 9 enter into either of the X-ray crystals 101 incidentally and transformed into signals. Backscattered electrons 4 are also emitted from the specimen 9, which must be eliminated because they cause noise if they enter the X-ray crystals 101.
In the collimator structure shown in FIG. 21, on the side walls of each passage hole for leading characteristic X-rays to the X-ray crystals 101, a pair of permanent magnets 270, 270 are installed to face toward each other across the passage space. A magnetic field 17 generated by these permanent magnets 270, 270 turns the above backscattered electrons 4 inward so that the electrons will strike against the beryllium plate 72 so as to be absorbed thereon.
Meanwhile, to enhance the sensitivity of X-rays detection, it is effective to detect X-rays from the specimen under measurement from a large solid angle of detection. The solid angle of detection is determined by the distance from the specimen to the X-ray crystal and the X-ray sensitive area of the crystal. The shorter the distance, and the larger the sensitive area, the solid angle will be larger.
The image of a specimen observed by an electron microscope is provided by detecting secondary emissions of electrons and imaging processing.
FIG. 22 is a sectional view of an X-ray detector equipped with a conventional electron trap 300 for explaining the relation between the detector and a leakage magnetic field. Reference numerals 230 and 231 denote permanent magnets, reference numeral 11 denotes a X-ray path hole, 25 denotes a leakage magnetic field, 9 denotes a specimen, 10 denotes a point of electron irradiation, 8 denotes an X-ray window, 16 denotes a cover, and 101 denotes an X-ray crystal.
In the conventional electron trap, the leakage magnetic field exists in the axial direction of the electron trap 300 as shown in FIG. 22. Consequently, the shape of the cross section of the electron beam adjusted to a circular section is more deformed to a non-circular shape as the electron trap 300 as the traps get closer to the specimen due to the interaction between the electron beam and the leakage magnetic field. An aberration called an astigmatism occurs, which deforms the image obtained by secondary electron detection, and causes the imaging resolution to deteriorate.
The position of the point of electron irradiation 10 is determined by the interaction between the electron beam and the leakage magnetic field. Because the intensity of the leakage magnetic field 25 at this position changes as the electron trap gets closer to the specimen. Such a phenomenon shifts the point of electron irradiation 10, and consequently, the image of the secondary electrons shifts.
When the X-ray detector approaches the specimen 9, the image of the secondary electrons is more deformed and shifted as described above and thus correction is required. The correction becomes impossible at a certain distance between the detector and the specimen. Therefore, a limit is set to the distance to which the X-ray crystal 101 can approach the specimen 9, and it is difficult to set the solid angle of detection large.
Then, the diameter of the electron beam that is a main factor of determining the resolution of the image of the secondary electrons will be described below. FIG. 23 is a graph representing beam current versus beam diameter relation for an electron gun of a thermionic emission type (with a tungsten hairpin filament and a lanthanum hexaboride (LAB6) point cathode) and an electron gun of a field emission (FE) type.
The graph shown in FIG. 23 is extracted from a reference xe2x80x9cElectron Microscopy Technologyxe2x80x9d published by Tonomura Akira, Maruzen Co., Ltd., (1989) pp. 1-22. From this graph, it is apparent that the smaller the electron beam current, the smaller the beam diameter is. Normally, it is possible to observe an image of the secondary electrons if the beam current is at least 10xe2x88x9211 amperes. However, because of the limited distance between the X-ray detecting unit of the conventional X-ray detector and the specimen, the efficiency of X-ray detection is low. Thus, the beam of electrons to strike the analyte is generally about 2xc3x9710xe2x88x9210 amperes, which is greater than the current applied when observing an image of the secondary electrons.
When a larger beam current is used, the beam diameter becomes larger as evident from FIG. 23 such that the imaging resolution decreases when an image of the secondary electrons is observed. Consequently, there is a trade off between the X-ray analysis and the resolution of the image of the secondary electrons, and these operations are usually carried out separately. As such, the operating conditions of the electron microscope must be adjusted to take measurements for each operation, which is troublesome and time-consuming.
On the other hand, the illustrated X-ray detector using a plurality of X-ray detecting units (FIGS. 20 and 21), the Prior Art, does have one advantage that the detection sensitivity can be multiplied as the number of the units increases. It may be possible for such a detector to perform X-ray analysis and observing a high-resolution image of the secondary electrons at the same time. However, the collimator structure of the electron traps of such a detector has a center hole for allowing the electron beam to pass and strike the analyte. The application of such a commonly used electron trap in X-ray detectors (essentially comprising a single X-ray detecting unit) is a question.
Another problem is that the collimator generates a large leakage magnetic field because the collimator essentially includes the holder made of nonmagnetic tantalum metal and permanent magnets.
The object of the present invention is to provide an X-ray detector having an electron trap performing such that an image of the secondary electrons is little deformed and shifted even if the detector gets to a specimen very close, and the electron trap is easily applicable to the conventional X-ray detectors essentially comprising a single X-ray detecting unit.
An X-ray detector and a charged-particle apparatus of the present invention achieve the above object based on the constitution of the invention described below.
The invention in one aspect is an X-ray detector comprising a specimen stage on which a specimen is rested, a charged particle beam irradiation unit for sending a beam of charged particles to strike the specimen set on the specimen stage, an X-ray crystal for detecting X-rays from the specimen by transforming the X-rays into electronic signals, a first pair of permanent magnets located across the path of the X-ray, another two pairs of permanent magnets, each pair of opposite magnets located on two ends of one of the first pair of permanent magnets while the two pairs of permanent magnets generate a magnetic field to cancel a leakage magnetic field generated by the first pair of permanent magnets at the point of irradiation.
The invention in another aspect is the discussed X-ray detector having the first pair of permanent magnets with their inner surfaces of opposite polarities facing toward each other and the other two pairs of permanent magnets are located such that their inner surface polarities facing toward each other are opposite and alternate with the polarities of the adjacent one of the first pair of permanent magnets.
The invention in a third aspect is the foregoing X-ray detector in which the magnetic force of the magnetic poles of the first pair of permanent magnets is substantially equal to the sum of the magnetic forces of the magnetic poles of the other two pairs of permanent magnets.
The invention in a fourth aspect is the foregoing X-ray detector in which the opposite surface area across the path hole of the first pair of permanent magnets is approximately double the parallel surface area of each of the other two pairs of permanent magnets.
The invention in a fifth aspect is the foregoing X-ray detector in which the first pair of permanent magnets and the other two pairs of permanent magnets are, separated by spacers made of a non-magnetic material.
The invention in a sixth aspect is the foregoing X-ray detector in which each of the first group of permanent magnets consists of four pieces of adjacent magnets of equal dimensions, and each of the other two groups of permanent magnets consists of two pieces of magnets of the same dimensions as the magnets of the first group.
The invention in a seventh aspect is a charged-particle apparatus characterized by comprising a specimen stage on which a specimen is rested, a source of charged particles, a scanning coil which scans the specimen set on said specimen stage with a beam of primary charged particles, an objective lens making the beam of charged particles strike the specimen, an electronic signal counter located between the objective lens and the specimen stage for counting the electronic signals outputted from the crystal, a display unit which displays an image of the electronic signals, while synchronizing signals from the electronic signal counter with the scanning signals that are input to the scanning coil, an X-ray detector including a pair of first permanent magnets across an X-ray path, two pairs of permanent magnets each pair of opposite polarities of which are located on one end of the first pair of external magnets along a direction parallel with the X-ray path. The two pairs of permanent magnets generate a magnetic field which cancels a leakage magnetic field generated by the first pair of permanent magnets at the specimen position, and a mechanism for moving/inching a charged particle beam irradiation relatively to the position of the charged particle trap.
The invention in an eighth aspect comprises an X-ray crystal, a first pair of permanent magnets placed above and below an X-ray path hole placed in front of said X-ray crystal such that their inner surfaces of opposite polarities facing toward each other across the path hole, a second pair of permanent magnets each placed on one end of one of said first pair of permanent magnets, and a third pair of permanent magnets each placed on one end of the other of said first pair of permanent magnets. The first pair of permanent magnets maybe divided into a plurality of pieces.
The invention in a ninth aspect comprises a specimen stage on which a specimen is rested, a source of charged particles, a scanning coil which scans the specimen set on the specimen stage with a beam of primary electrons from the source of electrons and an objective lens making the beam of primary electrons strike the specimen, a secondary electron detector located over the specimen stage to detect secondary electrons emitted from the specimen, a display unit for displaying an image of the electronic signals generated by the crystal, while synchronizing signals from the electronic signal counter with scanning signals that are input to the scanning coil, an X-ray crystal for transforming the X-rays emitted from the specimen into electronic signals, a first pair of permanent magnets located above and below an X-ray path hole, a second pair of permanent magnets each placed on one end of one of the first pair of permanent magnets, and a third pair of permanent magnets each placed on one end of the other of the first pair of permanent magnets, and a mechanism for moving/inching a charged particle beam irradiation area relatively to the position of the first pair of permanent magnets, i.e. the charged particle trap.
Other and further objects, features and advantages of the invention will appear more fully from the following description.